The 29-year-old spent the last two seasons on loan at Granada where he scored 26 goals in 60 appearances.
Last season was largely disrupted by injuries and the Switzerland-born forward was limited to a small number of appearances.
But now the strujwe will link up with Gianfranco Zola's side and could be in contention to feature in their Capital One Cup tie against Wycombe this weekend.
Geijo becomes the sixth player loaned by the Hornets from fellow Pozzo family-owned clubs Udinese and Granada.
Defender Daniel Pudil and attacker Ikechi Anya have joined from Granada while midfielder Almen Abdi and forwards Steve Leo Beleck and Matej Vydra are on loan from Udinese.